Humble sailor sea swimming (5)
I believe the answer is:
abase
'humble' is the definition.
(synonyms)
'sailor sea swimming' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'swimming' is an anagram indicator (letters swim around into a new order).
'sea' anagrammed gives 'ase'.
'ab'+'ase'='ABASE'
(Other definitions for abase that I've seen before include "Belittle, degrade" , "Lower" , "Humble (oneself)" , "Cast down" , "what 11 [REPRIMANDS] are meant to do" .)